As the title suggests, what have you done in the past that you totally regret now or was a big ol waste of time. Here is mine
I was doing keyword research (a few years ago when I was a NOOB) and found a keyword getting a few hundred thousand exact searches a month. I spent a few years building the site and getting backlinks (this is when making money online was a hobby) When I finally hit the front page of google for the keyword I got about 100-200 visits a day to the site!! WTF I was very confused.
Later I found out about the exact radio button
Moral of the story: ALWAYS select exact on the google keyword tool when doing keyword research!!